1. Field of the Invention
The present invention relates to operations involving the connection and disconnection of threaded tubular members on a drilling rig.
2. Description of the Related Art
In the construction of oil or gas wells it is usually necessary to prepare extremely long drill pipes or strings. Due to the length of pipe required, sections or stands of pipe are progressively added to the pipe as it is lowered into the well from a drilling platform. In particular, when it is desired to add a section or stand of pipe the string is usually restrained from falling into the well by applying the slips of a spider located in the floor of the drilling platform. The new section or stand of pipe is then moved from a rack to the well centre above the spider. The threaded pin of the section or stand of pipe to be connected is then located over the threaded box of the pipe in the well and the connection is made up by rotation therebetween. An elevator is connected to the top of the new section or stand and the whole pipe string lifted slightly to enable the slips of the spider to be released. The whole pipe string is then lowered until the top of the section is adjacent the spider whereupon the slips of the spider are re-applied, the elevator disconnected and the process repeated.
The first stage of making up the threaded connection normally involves the use of a drill pipe spinner located above the joint between the tubulars. The pin of the section of tubular to be added to the string is introduced into the box at the top of the string of tubulars, and the new section is spun by the spinner so that most of the connection is made under low torque. During this operation the spider holding the string generally provides sufficient reaction torque to prevent the string being rotated as the new joint is screwed in.
To complete the joint a much higher torque is required and it is common practice to use a power tong to provide this. The power tong is located on the platform, either on rails, or hung from a derrick on a chain, and is positioned around the joint once the initial stage of spinning the new tubular is complete. A two tong arrangement is used: an active (or wrenching) tong supplies torque to the section of tubular above the threaded connection, while a passive (or back up) tong supplies a reaction torque below the threaded connection, and prevents it from rotating. Such a tong arrangement is shown in FIG. 1.
It is important to ensure that when the tongs are tightened onto the tubulars, the joint between the tubulars is located between the tongs so that neither tong can tighten onto both tubulars.
Traditionally, the only way to monitor the position of the tubulars and tongs and to ensure that the junction between tubulars is correctly located between the tongs has been for a wellbore operative to stand beside the tongs and confirm by eye that the tubulars are in the correct position. It is desirable to automate the procedure around the head of the wellbore as much as possible so that operatives do not have to stand in this location, as it is a particularly dangerous environment. It is known to use a sensor which is moved axially relative to a pipe connection system to detect the presence of a tool joint. However, such a mechanism can only detect large upsets and is not suitable for detecting couplings between threaded connectors (e.g. in tubing and casing). Furthermore, the mechanism operates slowly, requiring axial movement over a predefined distance.